Generating a scan velocity modulation signal

ABSTRACT

The processing unit ( 1 ) for generating a scan velocity modulation signal (DS) has an input for receiving a video signal (VS) and an output for supplying a modulation signal (DS). The unit ( 1 ) comprises a series connection of, successively, a coring circuit (C), a differentiator (D), and a limiter (L). The display device ( 16 ) comprises a cathode ray tube ( 10 ) and the processing unit ( 1 ).

The invention relates to a processing unit for generating a scanvelocity modulation signal the processing unit having an input forreceiving a video signal and an output for supplying the modulationsignal, the invention also relates to a display device comprising acathode ray tube having means for modulating a scan velocity of anelectron beam, and to a method of generating a scan velocity modulationsignal.

An embodiment of such a unit is known from GB 2 064 911 A. In the knownunit, the video signal is differentiated and then supplied to circuitrywhich performs coring as well as limiting. When the modulation signal isapplied to a scan velocity modulation coil of a cathode ray tube, theobtained sharpness improvement is not optimal because of the presence ofnoise with a relatively large amplitude. The known display deviceincludes the known processing unit and a cathode ray tube. The knownmethod of generating the modulation signal includes the steps ofsubjecting the video signal to the successive steps of differentiating,coring and limiting the amplitude. It is a disadvantage of the knownunit that noise with a relatively large amplitude is present in themodulation signal.

It is an object of the invention to provide a modulation signal whereinnoise is suppressed to a large extent. The invention is defined by theindependent claims. The dependent claims define advantageousembodiments.

The object is realized in that the series connection comprises,successively from the input to the output, the coring circuit, thedifferentiator, and the limiter. The high frequency noise componentspresent in the video signal with an amplitude below the threshold of thecoring circuit are substantially removed from the video signal in thecoring circuit. Consequently, these removed components are not enhancedby the subsequent differentiation. When applying the video signal to thecoring circuit before differentiation, the low frequency components withan amplitude larger than the threshold are not removed by the coringcircuit and remain present in the modulation signal. In the known unit,the differentiator reduces the amplitudes of most low frequencycomponents to a level below the threshold, so the subsequent coringcircuit substantially removes these components from the video signal.Consequently, an additional advantage of the processing unit accordingto the invention is that the low frequency components remain present inthe modulation signal to provide scan velocity modulation.

The coring circuit may comprise a diode circuit comprising a first diodeand a second diode coupled anti-parallel, a cathode of the first diodebeing coupled to an anode of the second diode. By supplying the videosignal via this diode circuit to the differentiator, the desired coringis obtained in a very cost-effective way. The forward voltage of thediodes determines the threshold of the coring circuit.

The differentiator may comprise a first capacitor, which is coupled inseries with the diode circuit. This is a cost-effective way of realizingthe differentiator. Moreover, when the coring circuit and thedifferentiator are coupled in series, the coring circuit still preventsthe components with an amplitude below the threshold from reaching thedifferentiator.

It is advantageous if the amplifier has a transistor having a controlterminal as a negative input, a first main terminal as the positiveinput, and a second main terminal as an amplifier output. This amplifierhas a low input impedance, which is required to achieve adequatedifferentiation via the capacitor.

The limiter may comprise a series connection of another capacitor andanother diode circuit comprising a third diode and a fourth diode, theseries connection being coupled between a second reference voltagesource and the amplifier output. The forward voltage of the diodesdetermines the maximum amplitude of the modulation signal, being theoutput voltage of the limiter.

It is advantageous if the diodes are zener diodes. The desired limitingthreshold can be selected by choosing zener diodes with a zener voltagesubstantially equal to the desired threshold.

It is advantageous if a converter is present for converting themodulation signal into a drive current for the means for modulating thescan velocity; an input of the converter is coupled to the output of theunit; and an output of the converter is coupled to the means formodulating the scan velocity. The converter converts, for example, themodulation signal, being a voltage, into a current proportional to themodulation signal. This current is supplied to the means for modulatingthe scan velocity, usually being a coil mounted on the cathode ray tube.The current through the coil influences the speed of the deflection ofthe electron beam of the cathode ray tube.

These and other aspects of the invention are apparent from and will beelucidated with reference to the drawings, in which:

FIG. 1 shows a block diagram of the processing unit according to theprior art;

FIG. 2 shows a block diagram of the processing unit according to theinvention;

FIG. 3 shows a block diagram of the display device according to theinvention; and

FIG. 4 shows a circuit diagram of a part of the display device accordingto the invention.

Equivalent elements in the Figures have been given the same referencenumerals.

The prior-art processing unit 1 for generating a scan velocitymodulation signal in FIG. 1 comprises a differentiator D, a coringcircuit C and a limiter L, in that order, from the input to the output.The video signal VS at the input of the unit 1 is differentiated by thedifferentiator D and then applied to a coring circuit C that removes thesmall amplitude components in the differentiated video signal. As asubsequent step, or simultaneously, the limiter limits the maximumamplitude of the differentiated signal, resulting in the modulationsignal DS at the output of the unit 1. After differentiating, the lowfrequency components of the video signal VS are reduced in amplitude. Ifthe amplitude is reduced below the threshold of the coring circuit C,then these components are removed. Consequently, the modulation signalwill not generate the desired scan velocity modulation for these lowfrequency components. Moreover, the high frequency components of thenoise, present in the video signal, are amplified by the differentiatorD. This results in undesired, large amplitudes of the high frequencynoise components in the modulation signal DS.

In the unit 1 according to the invention, as shown in FIG. 2, thesequence of the elements is different from the prior art. First, thevideo signal VS is applied to the coring circuit C. The coring circuit Cremoves the small amplitude components of the video signal VS below apredetermined threshold. These components have to be removed becauseapplying scan velocity modulation for these components would result inan exaggerated correction of these amplitudes. At the same time anynoise, present in the video signal VS, including its high frequencycomponents, is suppressed as long as the amplitude of the noise is belowthe predetermined threshold. A considerable part of the noise spectrumis located in the upper part of the frequency spectrum of the videosignal VS. By removing these high frequency noise components in thecoring circuit C, it is avoided that these high frequency components arepassed to the next stage. Secondly, the signal coming from the coringcircuit C is amplified in the differentiator, which is equivalent to ahigh-pass filter, resulting in large undesired noise components in themodulation signal DS. Thirdly, the limiter L limits the amplitude of themodulation signal DS to a maximum amplitude, resulting in a modulationsignal DS providing adequate scan velocity modulation.

The block diagram of FIG. 3 shows the processing unit 1 receiving thevideo signal VS. The unit 1 supplies the driving signal DS to aconverter 14. The converter 14 is connected to means 12 for modulatingthe scan velocity, being, for example, a scan velocity modulation coil12 mounted on a cathode ray tube 10. If the modulation signal DS is avoltage waveform, the converter 14 converts the modulation signal DSinto a current flowing through the coil 12. The current through the coil12 generates an electromagnetic field, which modulates the velocity of adeflected electron beam in the cathode ray tube 10. If required, theconverter 14 also functions as an amplifier/buffer stage for themodulation signal DS.

In FIG. 4 the video signal VS is applied to the coring circuit C,comprising two anti-parallel connected diodes D1, D2. The differentiatorD, formed by capacitor C1, is connected in series with the coringcircuit. The cathode of the first diode D1 is connected to the anode ofthe second diode D2. If buffering is necessary to provide a low outputimpedance voltage drive, the video signal VS is supplied to the coringcircuit C via a buffer stage, comprising a first transistor Q1 andresistor R0. The output of the series connection of the diodes D1, D2and the capacitor C1 is connected to the emitter of a second transistorQ2. The emitter of the second transistor Q2 is connected via thirdresistor R3 to a reference voltage source V1, which can be ground level.A first resistor R1 is connected between the collector and the base ofthe second transistor Q2. A second resistor R2 and a second capacitor C2are connected in parallel between the base of the second transistor Q2and the first reference voltage source V1. The collector of the secondtransistor Q2 is also connected to a second reference voltage source V2via a fourth resistor R4. A series connection of another capacitor C3and another diode circuit, including at least a third diode D3 and afourth diode D4, is connected Parallel to the fourth resistor R4. Thethird diode D3 and fourth diode D4 may be connected anti-parallel, whilean anode of one of these diodes D3; D4 may be connected to the cathodeof the other diode D4; D3. Alternatively, each diode D3; D4 might berealized by connecting two diodes in series D3, D5; D4, D6. A furtheralternative is to use two zener diodes D3, D4 depending on the desiredvoltage range. Still further alternatives are possible, such ascombinations of the mentioned diode circuits. At the collector of thesecond transistor Q2, the modulation signal DS is present for drivingthe converter 14.

The unit 1 operates as follows. At the positive excursions of the videosignal VS, a forward current flows through the first diode D1 andcapacitor C1 to the resistor R3. During a subsequent negative goingexcursion, a reverse current starts flowing from the emitter of thefirst transistor Q1 via second diode D2 and first capacitor C1 towardsthe input of the unit 1, when the video signal VS has dropped by anamount equal to the sum of the forward voltages of the first diode D1and second diode D2. At the next positive excursion, the forward currentstarts to flow again, when the positive excursion again exceeds the sumof the forward voltages. Diodes usually have a forward voltage of about0.7 V, so the sum of the forward voltages is about 1.4 V. The result isthat only excursions of the video signal VS, which are at least 0.7 Vabove a reference value or at least 0.7V below the reference value, arepassed on to the emitter of the second transistor Q2. In this way, thedesired coring is obtained. The capacitor C1, receiving at one terminalvia the coring circuit the video signal VS from a low output impedancevoltage source, and having its other terminal connected to a low inputimpedance input of an amplifier, provides the differentiation.

The second transistor Q2 is arranged as an amplifier with a feedbackarrangement comprising the first resistor R1, the second resistor R2 andthe second capacitor C2. Such an arrangement is well known, and willtherefore not be further elucidated. The excursions of the output signalof the amplifier, present at the collector of the second transistor Q2,are limited by the series connection of the other capacitor C3 and theother diode circuit comprising, for example, two anti-parallel connecteddiodes D3, D4. If an excursion of this output signal exceeds the forwardvoltage of the concerned one of the two diodes D3; D4, this diode D3; D4starts to conduct, thereby preventing any further excursion of theoutput signal, being the modulation signal DS. Consequently, the twodiodes D3, D4 act as a limiter circuit, limiting the output voltageswing to the sum of the forward voltages of the two diodes D3, D4.

In the above example, the swing is approximately 1.4 V. If a largerswing is desired, more than one diode can be connected in series, asshown in FIG. 4. Alternatively, the two diodes can be replaced by zenerdiodes having a zener voltage corresponding to the desired swing.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Use ofthe article “a” or “an” preceding an element or steps does not excludethe presence of a plurality of such elements or steps. The invention canbe implemented by means of hardware comprising several distinctelements, and by means of a suitably programmed computer. In the deviceclaim enumerating several means, several of these means can be embodiedby one and the same item of hardware. The mere fact that certainmeasures are recited in mutually different dependent claims does notindicate that a combination of these measures cannot be used toadvantage.

1. A processing unit (1) for generating a scan velocity modulationsignal (DS), the processing unit having an input for receiving a videosignal (VS), and an output for supplying the modulation signal (DS),characterized in that the processing unit comprises, successively fromthe input to the output, a coring circuit (C), a differentiator (D), anda limiter (L).
 2. A processing unit (1) as claimed in claim 1,characterized in that the coring circuit (C) comprises a diode circuit,having a first diode (D1) and a second diode (D2) coupled anti-parallel,a cathode of the first diode (D1) being coupled to an anode of thesecond diode (D2).
 3. A processing unit (1) as claimed in claim 2,characterized in that the differentiator (D) comprises: a firstcapacitor (C1), which is coupled in series with the diode circuit (D1,D2).
 4. A processing unit (1) as claimed in claim 3, characterized inthat an amplifier (Q2, R1, R2, R3, R4) is present having a positiveinput coupled to an output of the coring circuit (C).
 5. A processingunit (1) as claimed in claim 4, characterized in that the amplifier (Q2,R1, R2, R3, R4) has a transistor (Q2) having a control terminalcorresponding to a negative input of the amplifier (Q2, R1, R2, R3, R4),a first main terminal as the positive input, and a second main terminalas an amplifier output.
 6. A processing unit (1) as claimed in claim 5,characterized in that the limiter (L) comprises a series connection ofanother capacitor (C3) and another diode circuit comprising a thirddiode (D3) and a fourth diode (D4), the series connection being coupledbetween a second reference voltage source (V2) and the amplifier output.7. A processing unit (1) as claimed in claim 6, characterized in thatthe third (D3) and the fourth diode (D4) are zener diodes coupledanti-parallel.
 8. A display device (16) comprising a cathode ray tube(10) having a processing unit (1) as claimed in claim 1 for generating ascan velocity modulation signal (DS); and means (12) for modulating ascan velocity of an electron beam coupled to the output of the unit (1).9. A display device (16) as claimed in claim 8, characterized in that aconverter (14) is present for converting the modulation signal (DS) intoa drive current for driving the means (12) for modulating the scanvelocity, the converter (14) having an input which is coupled to theoutput of the unit (1), and an output which is coupled to the means (12)for modulating the scan velocity.
 10. A method of generating a scanvelocity modulation signal (DS) wherein a video signal (VS) is subjectedto the successive steps of: coring; differentiating; and limiting anamplitude of the video signal (VS).